1. Technical Field
The present invention relates to storage batteries configured to increase the surface-to-weight ratio of active material exposed to electrolyte, to increase ampere-hour (Ah) capacity per kilogram (kg) of weight, to reduce the internal resistance of the battery, and to decrease cell deterioration by providing a uniform plate-electrode construction.
2. Description of the Related Art
In conventional batteries, such as lead-acid and alkaline batteries, solid active materials, such as spongy lead, lead peroxide and lead sulfate, are provided in the form of pastes supported by conductive grids on cathode and anode plates immersed in electrolyte. The negative and positive plates are rigidly attached to negative and positive cell electrodes respectively. Such conventional batteries are subject to inherent problems which limit their performance.
a. Conventional Lead Acid Batteries
Conventional lead-acid batteries typically have a number of cells connected in tandem. Each cell is separated from its adjacent cell by a solid wall so that the electrolyte from one cell cannot communicate with the electrolyte in the adjacent cell. Each cell further includes a multiplicity of positive and negative plates (typically 7 to 37 plates) which are sequentially positioned and firmly attached to respective positive and negative lead electrodes by lead electrode interconnections or buses. Each plate has a grid lattice made of antimonial lead, and active solid materials which are pressed onto the grid lattice. The solid active materials supported by the grids are in the form of a powder mixed with cohesive substances to form pastes. In lead-acid batteries, the active material in the charged state for the positive plate is lead peroxide and the active material in the charged state for the negative plate is lead. It should be noted that the active materials in the discharged state for both positive and negative plates is lead sulfate (PbSO.sub.4). The grid is provided to support the paste, and to conduct electricity between the solid active material and the electrodes of the cell. Insulating separators permeable to electrolyte ions are positioned between opposite polarity plates to prevent the plates from making electrical contact (i.e., from shorting).
One drawback associated with conventional lead-acid batteries is that the non-active lead which makes up the grids, the electrodes and the electrode buses, account for approximately half the weight of the battery without contributing to its ampere-hour (Ah) capacity, i.e., the overall active surface-to-weight ratio of the battery is low.
A second drawback of conventional lead-acid batteries is that only a small portion of the electrolyte is physically contained between the plates, where it can easily exchange ions with the solid active materials. To illustrate, as the plates and separators are pressed together, they limit the freedom of movement of the electrolyte ions therebetween. During fast discharges, the allowable density change in the electrolyte that is located between the plates is quickly consumed at only a fraction of the total ampere-hour discharge capacity of the battery.
A third drawback associated with conventional lead-acid batteries is the relatively high internal resistance. Various factors account for the high internal resistance. For example, some of the internal resistance is due to the collision of electrolyte ions as they try to pass through the separators and through the outer particle layers to reach particles deep within the active paste. A major contributing factor to the internal resistance is an effective resistance, which is a consequence of multiple contact resistance in tandem. Multiple contact resistance is defined as the series resistance encountered by the electrons as they move from particle to particle within the paste to reach the grid. To illustrate, the solid active materials are typically in the form of fine powder mixed with cohesive agents and other substances known as expanders, which also act as a conductor for electrons to travel along as the electrons move to or from particles located away from the grid. As a result, the effective resistance between an active particle and the grid varies depending on the distance of the particle from the grid. The effective resistance can range between a few ohms for particles located in close proximity to the grid, and several thousand ohms for particles located far from the grid. While the overall effective resistance is reduced by the high number of parallel paths the electrons may travel along, the effective resistance still remains high enough for conventional lead acid batteries to lose as much as 30% to 40% of their energy in terms of thermal losses. The expression Wr=I.sup.2 R represents the energy losses, where I represents the battery current during charge or discharge expressed in amperes, R is the battery internal resistance in ohms and Wr represents the energy lost expressed in watts.
The internal resistance also affects the output potential of the battery during operation by introducing a voltage drop, Vr=IR. Under high rates of discharge, the internal voltage drop substantially increases to the extent that the minimum output voltage needed by the load cannot be reached, while substantial energy still remains in the battery.
Another drawback in conventional lead-acid batteries is their relatively short life. Various reasons account for the short life span of such batteries. For example, one reason is the oxidation of the positive plate grid material which occurs when the battery is charged. As a result, the strength and electrical conductivity capabilities of the positive plate grid deteriorates to the point where the plate grid eventually collapses. Deterioration of the plate grid occurs more rapidly in the region where the plates having the same polarity are connected together via the lead electrode bus. In addition, at the points of contact between plates, especially the positive plates, gradual oxidation of the contact surfaces introduces high contact resistance in the electric conduction path. Other reasons contributing to a battery's deterioration is the shedding of the active paste particles during high rates of charge and discharge, and the creation of disruptive hydrogen and oxygen bubbles, especially during overcharge, which cause paste particles to dislodge. The dislodged particles which are shed can accumulate at, for example, the bottom of the battery and cause electric shorts between two opposite polarity plates.
b. Conventional Alkaline Storage Batteries
Conventional alkaline storage batteries are the nickel-iron cell (Edison cell), the nickel cadmium cell (Junker cell), and the zinc-manganese dioxide cell. Such alkaline storage batteries also suffer high internal resistance induced problems which make the batteries unsuitable for certain applications.
For example, in the Edison battery, the active materials are nickel oxyhydrate (NiOOH) as the positive active material, finely divided iron as the negative active material, and approximately 21% potassium hydroxide (KOH) as the electrolyte. During discharge, oxygen is transferred, via the electrolyte, from the positive active material to the negative active material. Even though the electrolyte acts only as an oxygen transfer agent, its viscosity does affect the internal resistance of the cell, indicating that a shorter path for the oxygen ions through the electrolyte would be desirable. In addition, the contact resistance between the iron particles, multiplied by the large number of such particles through which the electrons must travel to reach the electrodes, also contributes to the internal resistance of the battery. As in lead-acid batteries, the internal resistance in conventional alkaline batteries causes an internal voltage drop proportional to the battery current and a heat loss proportional to the square of such current.
c. Quasi-Fluid Batteries
Another type of storage battery is a quasi-fluid storage battery which provides the active materials in terms of thin-walled convoluted globules. The quasi-fluid storage batteries utilizing thin-walled convoluted globules are described in U.S. Pat. No. 4,735,873. While implementation of the globules is an effective configuration for certain uses, other uses require more efficient cell configurations.
Therefore, a need exists for an efficient battery cell configuration which increases the surface-to-weight ratio of active material exposed to electrolyte and which enhances the electrical connection between the active material and the cell respective electrode.